This invention relates to a device and method for filtering out undersirable components from an electric signal indicative of a weight value (hereinunder referred to as "weight signal").
The weight signal produced from a weighing device such as strain gauge load cell or force balance generally includes various undersirable components attributable to inherent vibration of the device, loading shock and like in addition to a necessary component indicative of the true weight of product to be weighed. In order to remove such undesriable components, an arrangement such as shown in FIG. 1 has been used, for example, as disclosed in U.S. Pat. No. 4,484,146. As shown, the weight signal from a weighing device 1 is passed through an analog filter 2, amplified by an amplifier 3 and converted into digital form by an analog-to-digital (A/D) convertor 4 for processing. An active filter having a low-pass characteristic, such as Bessel or Butterworth filter, has been used as the analog filter 2.
However, it has been difficult to completely remove the undesirable components by such an analog filter without effecting the response time of the weighing device, and it has been a general practice to add a mechanical vibration absorber. However, this additional mechanical component is undesirable since it complicates the mechanical structure and therefore the machine service and maintenance. Accordingly, it has been desired to remove the undesirable components by purely electrical means.
FIG. 2(a) shows a frequency characteristic of a typical load cell weigher including no mechanical damper, in which the transfer function of the weigher has a resonant peak in some frequency range. In the drawing, curve 5 applies to an unloaded weighing device and curve 6 applies to a loaded weighing device. This means that the peak frequency moves with the weight of product. Such movement is caused not only by load variation but also by mechanical resonance attributable to the device structure. However, such movement can be contained in a specific frequency band "b" as shown by suitably selecting its bandwidth. In order to remove the undesirable oscillatory components, it is necessary to provide some means for obtaining sufficient attenuation in this b-band. The next higher frequency band "c" may also be invaded by some unwanted frequency components attributable to mechanical shock applied, for example, when the device is loaded with product or when the hopper gates are opened. Therefore, it is also desirable to apply some attenuation to the c-band. Thus, the desired attenuation characteristic of vibration damping means will be as shown in FIG. 2(b). In the drawing, the amount of attenuation is indicated by the length of arrows.